The invention relates to a method for operating a fuel system of an internal combustion engine, wherein the fuel system has a fuel tank and a ventilation device for ventilating the fuel tank which ventilation device has at least one ventilation valve. The invention further relates to a fuel system.
Methods of the aforementioned type are known from the state of the art. The corresponding fuel system is for example assigned to a motor vehicle or a drive system respectively, of the motor vehicle. The drive system has in particular at least one internal combustion engine and is for example configured as hybrid drive system, i.e. has the internal combustion engine as well as at least one electric machine, wherein the internal combustion engine and the electric machine at least temporarily cooperate to generate a drive torque of the drive system. The internal combustion engine is supplied with fuel by the fuel tank by the fuel system. Oftentimes, a volatile hydrocarbon is used as fuel for example gasoline. The fuel tank therefore normally contains a volume of liquid fuel as well as gaseous fuel which in particular accrues above the liquid fuel. The fuel tank can be a closed tank, in particular a pressure tank or a partially closed in particular also a pressure free tank. The closed tank is in particular used to reduce emissions.
Due to temperature fluctuations of the fuel, for example caused by changes of the ambient temperature, pressure fluctuations can occur in the fuel tank. For this reason, the ventilation device is assigned to the fuel tank. It serves for ventilating the fuel tank. In this way, excessive pressure in the fuel tank can be reduced by the ventilating device. For this purpose the ventilation device ventilates the fuel tank for example by a ventilation line. During ventilation, gaseous as well as liquid fuel can escape the fuel tank through the ventilation device or the ventilation line. The ventilated fuel thus is first present as a mixture of gaseous and liquid fuel. This is particularly the case when the fuel tank is ventilated at a high internal pressure of the fuel tank. As a result of the high pressure or the great pressure difference between the pressure inside the fuel tank and the pressure outside of the fuel tank, the ventilated fuel has high flow velocities, which causes liquid fuel to be carried along by the gaseous fuel.
The gaseous fuel can readily be supplied to the internal combustion engine or its intake system, wherein a fuel accumulator which is preferably configured as activated carbon filter can be provided between the fuel tank and the internal combustion engine. The fuel accumulator has the purpose to temporarily store gaseous fuel, i.e. to take up gaseous fuel when unused gaseous fuel is present and to give off gaseous fuel as soon as the gaseous fuel can be discharged into the internal combustion engine. However, liquid fuel must not enter the fuel accumulator or the internal combustion engine.
For this reason, the ventilation device can have at least one separation device which serves for separating gaseous and liquid fuel. The separation device thus has the purpose to prevent the transfer of liquid fuel from the fuel tank through the ventilation device into the internal combustion engine. The separation device separates liquid fuel and allows gaseous fuel to pass. The separated liquid fuel enters a temporary accumulator of the separation device. The term temporary accumulator does not mean that a (temporary) accumulation of the liquid fuel is indeed provided for. Rather, the liquid fuel can be discharged directly out of the temporary accumulator or the separation device preferably in the direction of the fuel tank. Here, however, a rise of the fill level of the temporary accumulator may result, for example due to a limitation of the discharge volume flow, in particular by a line cross section or the like. The separated liquid fuel thus temporarily cannot be discharged at the same rate as it enters the temporary accumulator. Of course, a temporary storage of the liquid fuel, for example over a defined period of time can also be realized.
When operating the fuel system, the amount of liquid fuel present in the temporary accumulator or the separation device should be prevented from exceeding a threshold amount, i.e. from being greater than the fill level of the temporary accumulator, because this may impair the effectiveness of the separation device. The greater the amount of liquid fuel in the temporary accumulator, the greater is the risk that liquid fuel also exits the separation device together with the gaseous fuel and is carried along in the direction of the fuel accumulator or the internal combustion engine. For this reason the fuel delivery unit can be assigned to the separation device. The former is used to deliver liquid fuel out of the temporary accumulator, in particular in the direction of the fuel tank. The fuel delivery unit is usually configured as jet pump wherein fuel which is delivered by a fuel pump of the fuel system out of the fuel tank in the direction of internal combustion engine, is often used as operating means for the jet pump.
In known methods for operating the fuel system of the internal combustion engine, the ventilation of the fuel tank takes places by means of the ventilation device, usually based on the pressure inside the fuel tank and a temperature. This means that for ventilating the fuel tank the ventilation valve of the ventilation device is adjusted based on the internal pressure of the fuel tank and the temperature. However, this is only possible when the internal combustion engine is activated, because only in this case a control device which is assigned to the internal combustion engine or the fuel system respectively is activated. This control device serves for adjusting the ventilation valve for ventilating the fuel tank based on the pressure inside the fuel tank and the temperature. In phases of standstill, i.e. when the internal combustion engine and with this the control device, is deactivated the pressure inside the fuel tank is usually limited by at least one mechanical overflow valve which is configured for a typical pressure level. When the temperature in the fuel tank changes for example as a result of heat input by the internal combustion engine which is still hot from operation, or as a result of external influences, a pressure which correlates with the steam pressure of the fuel is established inside the fuel tank. When this pressure reaches or exceeds the typical pressure level and in particular a defined maximal pressure inside the fuel tank, the overflow valve opens for ventilating the fuel tank. A deactivated internal combustion engine in this context relates to an internal combustion engine at standstill, while an activated internal combustion engine is operated at least in the idle gear in particular provides a torque.
When the internal combustion engine is deactivated, the ventilation of the fuel tank thus can take place solely based on the pressure inside the fuel tank. However, oftentimes the permissible maximal pressure inside the fuel tank changes with the temperature, i.e. is dependent on the latter. In particular when the internal combustion engine is deactivated and the pressure inside the fuel tank can only be decreased via the overflow valve, it can thus occur that the fuel tank is exposed to a pressure inside the fuel tank which causes forces which exceed the strength of the fuel tank. This leads for example to unacceptable flow behavior or to undesired irreversible deformations of the fuel tank, in particular of a fuel tank shell.